Equilibria between pure solids and liquids

If the partial molar volumes of the condensed phase are negligibly small with respect to the molar volume of the gas phase, this becomes 

We see that under these conditions the value of the derivative becomes zero (a maximum) when xx = 2x2. The maximum then occurs when the composition of the condensed phase is that of the species A2B. When the partial molar volumes of the condensed phase are not negligibly small and when the nonideality of the gas is included, the maximum does not occur at this exact composition, but does occur very close to it [32]. 

In the second example we assume that the species in the gas phase are A, B, and AB, and wish to obtain an expression for (d In B ab d 1° P a)t The Gibbs-Duhem equations are 

The interest in this equation is that it affords the calculation of the composition of the condensed phase in equilibrium with the gas phase when the values of the derivative, yA, yB, and yAB can be determined [32]. 

Consideration of the condition of equilibrium applied to reactions between pure condensed phases, liquid or solid only, leads to some interesting 

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